1. Field of the Invention
This invention relates to a method of depositing tungsten upon a semiconductor substrate. More particularly, it relates to selective deposition of a thick layer of tungsten for use as an electrode or wiring for the semiconductor device.
2. Description of the Related Art
In semiconductor LSI (Large Scale Integrated Circuit) devices, polycrystalline silicon has been widely employed for fabricating electrodes and electrical wiring. However, the lowering of the electrical resistance of polycrystalline silicon is limited because it is a semiconductor. Therefore, a refractory material, such as tungsten, or tungsten silicide has been employed in place of the polycrystalline silicon. The fabrication of tungsten electrodes or wiring is schematically explained hereinafter referring to FIG. 1. The surface of a silicon semiconductor substrate element is coated with a masking film 2 formed from a phospho-silicate glass (referred to hereinafter as SiO.sub.2) material generally to a thickness of approximately 1 .mu.m. Masking film 2 is patterned using generally known lithography techniques, so that an opening in the form of contact hole 3 is provided at the portion of the substrate on which tungsten is to be deposited. The substrate 1 having the masking film 2 thereon is loaded into a conventional apparatus used for chemical vapor deposition (CVD). A reaction gas, such as a gaseous mixture of tungsten hexafluoride (referred to hereinafter as WF.sub.6) and hydrogen (referred to hereinafter as H.sub.2) gas, is fed into the CVD apparatus, and the gas pressure is reduced to the range of 0.1 to 0.5 Torr, and then the substrate 1 having the masking film 2 thereon is heated. Chemical reactions occur in accordance with the following relationships: EQU 2 WF.sub.6 +3 Si=2 W+3 SiF.sub.4 (1) EQU WF.sub.6 +3 H.sub.2 =W+6 HF (2)
The exposed silicon on the substrate 1 is active to reduce the WF.sub.6 and thus produce HF (hydrofluoric acid) and W (tungsten). The produced W is deposited upon the surface of the exposed portion 1' of the silicon substrate 1. The reducing reaction pursuant to relationship (1) does not take place in areas covered by the SiO.sub.2 masking film 2, and therefore little tungsten deposits on the masking film 2. Thus, deposition of tungsten occurs selectively. However, when deposited tungsten forms a film at the exposed portion 1' of the silicon substrate 1, such formed tungsten film prevents the silicon substrate from reacting further with WF.sub.6 in the reaction gas, and the thickness of the deposited tungsten film will generally be no greater than approximately 1000.ANG.. In order to continue supplying tungsten to the surface film, H.sub.2 is admixed with the WF.sub.6 in the reaction gas. H.sub.2 reacts in accordance with relationship (2) not only to produce additional thickness of tungsten on the previously deposited tungsten, but also to produce new tungsten nucleuses on the surface of the masking film 2. Such nucleuses will continue growing so as to form a second tungsten film that could imperil the function of the masking film by electrical isolation. Such additional film is also not easily removed. Therefore, the reaction in accordance with relationship (2) must be discontinued before such additional film is formed on the masking film 2. The result is that the thickness of the tungsten film 4 deposited on the exposed silicon substrate 1' cannot be more than about 3000.ANG. and typically is about 2000.ANG.. Such thickness is sufficient for the deposited tungsten 4 to perform as an electrical conductor. However, the large difference between the thickness of the masking film 2 (approximately 1 .mu.m) and of the deposited tungsten film 4 (approximately 2000.ANG.) causes problems during later processing aimed at fabricating reliably continuous aluminum wiring 5 thereon and connecting the deposited tungsten, i.e., an electrode, to an outside connection. In other words, the accuracy of the photo lithography process for depositing the aluminum film 5 on such a non-flat surface is, of course, diminished, and also, the sharp edges of the masking film 2 may cause discontinuities to occur during aluminum deposition. This problem becomes impermissibly serious in connection with the fabrication of VLSI (Very Large Scale Integrated Circuit) of sub-micron order. Therefore, methodology has been needed to facilitate fabrication of a thick tungsten film so as to present a substantially flat surface together with the surface of a masking film.